The present invention relates to a coated film. More particularly, it relates to a film having a novel coating layer with high transparency and surface gross, anti-blocking property and strong adherence to various facing materials.
Since a stretched polyester film has many excellent properties such as high mechanical strength, dimensional stability, planarity, smoothness, heat resistance, chemical resistance and transparency, it is used in wide variety applications, such as base film for magnetic recording media, packaging material, platemaking film, base for magnetic cards, and base for synthetic paper.
However, as its surface has high-degree crystalline orientation, the polyester film has the disadvantage of being low in adherence to inks, various types of coatings and facing materials.
As a solution to the problem of such low adherence, it has been proposed to apply various types of resin on the polyester film surface to provide an easily adhering coating layer. Especially a method in which such coating is carried out in the film production process (in-line coating) prevails for its economical advantage and its capability to provide the good film properties. In a typical instance of this method, coating is conducted at a point after stretching in the machine direction and before stretching in the transverse direction, and then stretching in the transverse direction and heat setting are performed.
In case where a coating layer is provided by using an easily adhering resin as said above, there tends to occur so-called “blocking”, a phenomenon of the films to stick to each other. Also, sufficient adhesion to the facing may not be obtained by merely providing a coating layer with such an easily adhering resin.
Incorporation of a crosslinking agent as a component of the coating layer has been proposed as a method for reducing the blocking tendency of the film and improving its adhesiveness. However, addition of a crosslinking agent in the coating layer composition usually prompts hardening of the formed coating layer, so that in case where stretching is carried out after coating as in the in-line coating system, the coating layer may be unable to follow stretching to cause fine cracks in the layer surface, giving rise to the problems such as improper adhesion to the facing and clouding of the coating layer due to scattering of light. Even when the base film is not transparent, there can arise the problem of reduced surface gloss.